Gardeners should recognize and encourage natural enemies

By ED PERRY

last updated: April 11, 2008 12:10:35 PM

In a recent column I discussed the importance of "natural enemies" in reducing the population of aphid pests in gardens. For those who may be unfamiliar with the term, natural enemies are those organisms that kill or otherwise reduce the numbers of plant pests. Important natural enemies of insect and mite pests include predators, parasites, and pathogens (disease-causing organisms).

Natural enemies called predators are organisms that attack kill, and feed on several to many other individuals (their prey) in their lifetime. Many beetles (lady beetles, predaceous ground beetles, etc.), true bugs, flies and lacewings are predators of various pest mites and insects. Most spiders are predators, feeding entirely on insects. Various species of predatory mites prey primarily on spider mites, which are major plant pest. Many amphibians, reptiles, birds and mammals (bats) also prey on insects.

Natural enemies known as parasites are organisms that live and feed in or on larger hosts. Insect parasites (more accurately called parasitoids) are smaller than their host and develop inside, or attach to the outside, of the host's body. Often only the immature stage, or larvae, of the parasite feeds on the host, and it kills only one host individual during its development. However, adult females of certain parasites (such as many wasps that attack scales and whiteflies) also become predators by feeding on their hosts. Most parasitic insects are either flies or tiny to medium-sized wasps. Parasitic wasps parasitize aphids and insect eggs, but do not sting people. The most common parasitic flies are in the family Tachinidae, and often resemble house flies. Their larvae are maggots that feed inside the host.

Pathogens considered natural enemies include certain bacteria, fungi, nematodes, protozoa and viruses that can infect and kill the host. Pests such as aphids, caterpillars and mites are sometimes drastically reduced by naturally occurring pathogens, usually under conditions such as prolonged high humidity or dense pest populations. Along with naturally occurring disease outbreaks, some beneficial pathogens are commercially available as biological or microbial pesticides. These include Bacillus thuringiensis (Bt), insect-infecting nematodes and granulosis viruses.

By properly managing your garden, you can favor and encourage natural enemies. Grow diverse plant species and learn to tolerate low populations of plant pests so that some food is always available to sustain predators and parasites. Plant a variety of flowering plants to provide natural enemies with nectar, pollen, and shelter throughout the growing season. The adult stage of many predaceous and parasitic insects feed only on pollen and nectar, so flowers are essential to their survival. Avoid overfertilizing and overwatering, which can cause sucking pests such as aphids to reproduce more rapidly than natural enemies can provide control. Also, because dust can interfere with natural enemies and favors pests such as spider mites, reduce dust by covering bare ground with coarse mulch or by hosing off plant foliage.

It's important to be able to recognize both pests and natural enemies in your garden in order to take full advantage of biological control. For example, many people do not recognize lady beetle larvae, which are very effective aphid predators. Sometimes aphid-eating syrphid fly larvae are confused with leaf-eating caterpillars. And often adult syrphid flies, commonly also called flower flies or hover flies, are mistaken for honey bees. To help you with identification, consult publications such as the Natural Enemies Handbook or the UC Integrated Pest Management Program Pest Notes series, both available at your local University of California Cooperative Extension Office, or at www.ipm.ucdavis.edu.

Ed Perry, a farm adviser with the UC Cooperative Extension, can be contacted at 525-6800 or ejperry@ucdavis.edu.

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